Will Not Boot Up

Hi, I am new to Macs after many years with PCs. I have an older iMac (white plastic) updated to Snow Leopard without any problems until now. It froze whilst downloading latest iPhone update through iTunes last night. I forced a shutdown with the button on back of screen.

Today, it will not boot up, help!

After pressing the button, it chimes, then grey apple logo and spinning ball. At that point I get a strange screen, see picture.

I have tried starting in safe mode and it runs without probelm, although there is a pattern beneath the cursor.

That's pretty clearly graphics. Try a PRAM reset. It's not likely, but it might clear it up. The reason it kinda works in safe mode is because the graphics drivers aren't loaded, so nothing special is being done on the graphics card. If it does the same thing booted from a known-good OS, then it's going to be your graphics card (if it's a model that has a separate card) or logic board (otherwise).

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